Interconnectivity of a multiplayer interconnection substrate greatly depends upon positions of vias which connect interconnection layers mutually. When designing interconnections of a multilayer interconnection substrate, therefore, it is important to dispose vias in optimum positions as far as possible.
In many cases, as long as approximate positions of vias are determined, the approximate route of its interconnection is uniquely determined. Therefore, an interconnection process includes the following stages in general:
(1) a stage of finding approximate positions of vias and approximate interconnection routes; and
(2) a stage of finding detailed interconnections
When determining approximate positions of vias, approximate positions of vias are determined to lower the degree of congestion as far as possible and thereby increase the interconnectivity in the subsequent detailed interconnection process. In other words, positions of vias are determined in a manner that the number of interconnections passing between vias does not exceed the number of passable interconnections which depends on the design rule.
A technique for finding appropriate interconnections while improving the via positions is also proposed (for example, Yoichi Tomioka, Atsushi Takahashi, “Routability Driven Modification Method of Monotonic Via Assignment for 2-layer Ball Grid Array Packages”, in Proceedings of the 2008 conference on Asia and South Pacific design automation, pp. 238-243, 2008).
If there is an interconnection for a high speed signal in an interconnection substrate having a power supply plane or a ground plane, then it is desirable that the power supply plane or the ground plane always passes right under the high speed signal interconnection in order to keep the characteristic impedance constant as far as possible (see, for example, Japanese Patent Laid-Open Pub. 2009-081534).
On the other hand, if signal interconnections are connected to each other via a via in an interconnection layer located above or below the power supply plane or the ground plane, an opening is provided through the power supply plane or the ground plane in order to insulate the via from the power supply plane or the ground plane (see FIG. 3B).
If a high speed signal line is interconnected in a position adjacent to the via, then the high speed signal passes over a hole formed through the power supply plane or the ground plane and it becomes difficult to keep the characteristic impedance constant.
For avoiding such a situation, it is necessary to make the interval between the via and the interconnection greater than an interval which is needed in a usual situation.
If all intervals between signal interconnections and vias are made large, however, then an interconnection area which is greater than needed is required and interconnectivity (degree of easiness of interconnection) remarkably falls.
It is ideal to make the interval between an interconnection and a via large only when a signal interconnection adjacent to the via is a high speed signal line. However, there has not been such an automatic interconnection technique until now.